Balanced positioning system for use in lithographic apparatus

ABSTRACT

A balanced positioning system for use in lithographic apparatus having a pair of balance masses which are supported so as to be moveable in at least one degree of freedom, such as Y translation. Oppositely directed drive forces in this degree of freedom act directly between the driven body and the balance masses to rotate the driven body about an axis perpendicular to the one direction. Reaction forces arising from positioning movements result in linear movements of the balance masses and all reaction forces are kept within the balanced positioning system.

[0001] This is a continuation application of U.S. application Ser. No.10/209,926, filed Aug. 2, 2002, which is a continuation application ofU.S. application Ser. No. 09/739,098, filed Dec. 19, 2000, which is nowU.S. Pat. No. 6,449,030, which claims priority from European Patentapplication No. 99310371.2, filed Dec. 21, 1999, the entire contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to balanced positioning systems.More particularly, the invention relates to such systems in lithographicprojection apparatus comprising:

[0004] a radiation system for supplying a projection beam of radiation;

[0005] a first object table for holding a mask;

[0006] a second object table for holding a substrate; and

[0007] a projection system for imaging an irradiated portion of the maskonto a target portion of the substrate.

[0008] 2. Discussion of Related Art

[0009] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, catadioptric systems,and charged particle optics, for example. The radiation system may alsoinclude elements operating according to any of these principles fordirecting, shaping or controlling the projection beam of radiation, andsuch elements may also be referred to below, collectively or singularly,as a “lens”. In addition, the first and second object tables may bereferred to as the “mask table” and the “substrate table”, respectively.Further, the lithographic apparatus may be of a type having two or moremask tables and/or two or more substrate tables. In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more stages while one ormore other stages are being used for exposures. Twin stage lithographicapparatus are described in International Patent Applications WO 98/28665and WO 98/40791, for example.

[0010] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, the mask(reticle) may contain a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(comprising one or more dies) on a substrate (silicon wafer) which hasbeen coated with a layer of photosensitive material (resist). Ingeneral, a single wafer will contain a whole network of adjacent targetportions which are successively irradiated via the mask, one at a time.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat once; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—which is commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned from International PatentApplication WO 97/33205.

[0011] In a lithographic apparatus, reactions on the machine frame toacceleration forces used to position the mask (reticle) and substrate(wafer) to nanometer accuracies are a major cause of vibration,impairing the accuracy of the apparatus. To minimise the effects ofvibrations, it is possible to provide an isolated metrology frame onwhich all position sensing devices are mounted, and to channel allreaction forces to a so-called force or reaction frame that is separatedfrom the remainder of the apparatus.

[0012] U.S. Pat. No. 5,208,497 describes a system in which the reactionof the driving force is channeled to a balance mass which is normallyheavier than the driven mass and which is free to move relative to theremainder of the apparatus. The reaction force is spent in acceleratingthe balance mass and does not significantly affect the remainder of theapparatus. However, the concept disclosed in U.S. Pat. No. 5,208,497 isonly effective for reaction forces in one direction and is not readilyextendable to systems having multiple degrees of freedom. Balance massesmoveable in three degrees of freedom in a plane are described in WO98/40791 and WO 98/28665 (mentioned above).

[0013] EP-A-0,557,100 describes a system which relies on activelydriving two masses in opposite directions so that the reaction forcesare equal and opposite and so cancel out. The system described operatesin two dimensions but the active positioning of the balance massnecessitates a second positioning system of equal quality and capabilityto that driving the primary object.

[0014] None of the above systems is particularly effective atcounteracting yawing moments which may be induced by adjustments of therotational position of the driven mass or because of misalignmentbetween the line of action of forces exerted on the driven body and itscenter of mass.

[0015] U.S. Pat. No. 5,815,246 discloses a positioning system having afirst balance mass free to move in an XY plane, i.e. to translate in Xand Y and rotate about axes parallel to the Z direction. To controlrotation of the first balance mass, a fly wheel, forming a secondbalance mass, is driven by a rotation motor mounted on the first balancemass to exert a counter-acting torque. Controlling rotation of the firstbalance mass therefore requires accurate control of the rotation and theflywheel. Any delay in this control or imbalance of the flywheel willcause vibration.

SUMMARY OF THE INVENTION

[0016] An object of the present invention is to provide an improvedbalanced positioning system for counteracting yawing moments in thedriven mass and preferably also force balancing in at least twotranslational degrees of freedom.

[0017] According to the present invention there is provided alithographic projection apparatus comprising:

[0018] a radiation system for supplying a projection beam of radiation;

[0019] a first object table for holding a mask;

[0020] a second object table for holding a substrate;

[0021] projection system for imaging irradiated portions of the maskonto target portions of the substrate; characterized by:

[0022] a balanced positioning system for positioning at least one ofsaid object tables and comprising:

[0023] first and second balance masses;

[0024] bearing means for supporting said first and second balance massesso as to be substantially free to translate in at least one direction;and

[0025] driving means for acting directly between said one object tableand said first and second balance masses to rotate said object tableabout an axis perpendicular to said one direction, said driving meansbeing arranged to exert linear forces on said first and second balancemasses in opposite directions to effect said rotation of said objecttable.

[0026] By providing two balance masses that can translate in at leastone direction, the torque required to drive the object table to adjustits rotational position, or to compensate for torques induced by otherdriving forces can be provided as the sum of two linearly acting forcesacting between the object table and the two balance masses. The reactionforces on the two balance masses will cause them to move linearly, whichcan easily be accommodated. In other words, the reaction to a torqueexerted on the driven object table is converted to translations of thetwo balance masses and no rotational movement of the balance massoccurs. It will be appreciated that if a rotational motion of the objecttable is combined with a linear motion, the net forces acting on eachbalance mass may be in the same direction, though different inmagnitude.

[0027] According to a yet further aspect of the invention there isprovided a method of manufacturing a device using a lithographicprojection apparatus comprising:

[0028] a radiation system for supplying a projection beam of radiation;

[0029] a first object table for holding a mask;

[0030] a second object table for holding a substrate; and

[0031] a projection system for imaging irradiated portions of the maskonto target portions of the substrate; the method comprising the stepsof:

[0032] providing a mask bearing a pattern to said first object table;

[0033] providing a substrate provided with a radiation-sensitive layerto said second object table;

[0034] irradiating portions of the mask and imaging said irradiatedportions of the mask onto said target portions of said substrate;characterized in that:

[0035] at least one of said object tables is positioned using apositioning system which includes first and second balance masses freeto move in at least one direction and drive means acting between saidone object table and said balance masses; and

[0036] during or prior to said irradiating step said one object table isrotated by exerting oppositely directed forces between it and said firstand second balance masses.

[0037] In a manufacturing process using a lithographic projectionapparatus according to the invention a pattern in a mask is imaged ontoa substrate which is at least partially covered by a layer ofenergy-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallisation,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

[0038] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

[0039] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation or particleflux, including, but not limited to, ultraviolet radiation (e.g. at awavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), EUV, X-rays,electrons and ions.

[0040] Embodiments of the present invention are described below makingreference to a Cartesian coordinate system with axes denoted X, Y and Zin which the XY plane is parallel to the nominal substrate and reticlesurfaces. The notation Ri is used to denote rotation about an axisparallel to the I direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

[0042]FIG. 1 depicts a lithographic projection apparatus according to afirst embodiment of the invention;

[0043]FIG. 2 is a plan view of the reticle stage of the apparatus ofFIG. 1;

[0044]FIG. 3 is an end view of the reticle stage of the apparatus ofFIG. 1;

[0045]FIG. 4 is a diagram of a servo control mechanism used in the firstembodiment of the present invention;

[0046]FIG. 5 is a plan view of the reticle stage of a second embodimentof the invention;

[0047]FIG. 6 is an end view of the reticle stage of the secondembodiment of the invention;

[0048]FIG. 7 is a plan view of the reticle stage of a third embodimentof the invention;

[0049]FIG. 8 is an end view of the reticle stage of the third embodimentof the invention; and

[0050]FIGS. 9 and 9A show a cable ducting device useable in embodimentsof the invention.

[0051] In the drawings, like reference numerals indicate like parts.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0052] Embodiment 1

[0053]FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

[0054] a radiation system LA, IL for supplying a projection beam PB ofradiation (e.g. UV or EUV radiation, x-rays, electrons or ions);

[0055] a first object table (mask table) MT for holding a mask MA (e.g.a reticle), and connected to first positioning means for accuratelypositioning the mask with respect to item PL;

[0056] a second object table (substrate table) WT for holding asubstrate W (e.g. a resist-coated silicon wafer), and connected tosecond positioning means for accurately positioning the substrate withrespect to item PL;

[0057] a projection system (“lens”) PL (e.g. a refractive orcatadioptric system, a mirror group or an array of field deflectors) forimaging an irradiated portion of the mask MA onto a target portion C(comprising one or more dies) of the substrate W.

[0058] As here depicted, the apparatus is of a transmissive type (i.e.has a transmissive mask). However, in general, it may also be of areflective type, for example.

[0059] The radiation system comprises a source LA (e.g. a Hg lamp,excimer laser, an undulator provided around the path of an electron beamin a storage ring or synchrotron, or an electron or ion beam source)which produces a beam of radiation. This beam is caused to traversevarious optical components comprised in the illumination system IL,—e.g. beam shaping optics Ex, an integrator IN and a condenser CO—sothat the resultant beam PB has a desired shape and intensity throughoutits cross-section.

[0060] The beam PB subsequently intercepts the mask MA which held on amask table MT. Having traversed the mask MA, the beam PB is caused totraverse the lens PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the interferometric displacementmeasuring means IF, the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of the beamPB. Similarly, the first positioning means can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library. The referencesigns M1, M2 correspond to reticle alignment marks and the references P1and P2 correspond to wafer alignment marks. These marks are used toalign the wafer and the reticle respective to each other. In general,movement of the object tables MT, WT can be realized with the aid of along stroke module (coarse positioning) and a short stroke module (finepositioning), which are not explicitly depicted in FIG. 1.

[0061] The depicted apparatus can be used in two different modes:

[0062] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected at once (i.e. a single“flash”) onto a target portion C. The substrate table WT is then shiftedin the x and/or y directions so that a different target portion C can beirradiated by the beam PB;

[0063] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash”.Instead, the mask table MT is movable in a given reference direction(the so-called “scan direction”, e.g. the Y direction) with a speed v,so that the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the lens PL (typically, M=¼ or ⅕). In this manner, a relatively largetarget portion C can be exposed, without having to compromise onresolution.

[0064]FIGS. 2 and 3 show the reticle (mask) stage of the firstembodiment of the invention in greater detail. The mask MA (not shown inFIG. 2), whose pattern is to be imaged onto the wafer, is held on masktable MT. To accommodate the scan mode of the apparatus the mask must bepositioned accurately over a relatively wide range of movement (stroke)in the Y direction but only over much smaller ranges of movement in theother degrees of freedom. This large Y-direction stroke, as well as amore limited stroke in the X-direction and some Rz movement, is effectedby the long stroke (coarse positioning) module described below. Finepositioning in all six degrees of freedom is accomplished by shortstrokeposition actuators included in the mask table.

[0065] Mask table MT depicted in FIGS. 2 and 3 is intended for use withtransmissive masks which means that the space above and below it must bekept clear. Accordingly, mask table MT is supported from two balancemasses 20, 30 positioned either side of a clear space extending in theY-direction. In the present embodiment, three beams 11, 12, 13, whichextend transversely from mask table MT, are provided for this purposebut the beams may alternatively be formed integrally with the body ofmask table MT or the mask table may itself extend over the balancemasses 20, 30. Balance masses 20, 30 have parallel planar upper surfacesagainst which table bearings 14, 15, 16 provided on the ends of beams11, 12, 13 act to support the mask table. Table bearings 14, 15, 16allow mask table MT to move in the XY plane relative to balance masses20, 30 substantially without friction. Table bearings 14, 15, 16 may,for example, be gas bearings. Z-direction actuators may also be includedin these bearings for coarse positioning in Z, Rx and Ry.

[0066] Balance masses 20, 30 are supported by substantially frictionlessZ-bearings 21, 22, 23, 31, 32, 33 on parallel rails 40, 50 which extendin the Y-direction and may be part of or connected to the main machineframe, or base plate, BP. Rails 40, 50 have substantially flathorizontal upper surfaces 41, 51 against which Z-bearings 21, 22, 23,31, 32, 33 act so that the balance masses 20, 30 are free to move in theY-direction over a relatively wide range of motion. Z-bearings 21, 22,23, 31, 32, 33 may be compliant, i.e. have a low stiffness, in theZ-direction so that the balance masses 20, 30 are also substantiallyfree to move in the Z-direction, though over a much smaller range ofmovement. Freedom for the balance masses to move in the X-direction maybe similarly provided by compliant X-bearings 24, 25, 34, 35 actingagainst substantially planar vertical walls 42, 52 of the rails 40, 50.X-bearings 24, 25, 34, 25 may be preloaded or opposed pad bearings toexert forces in both directions. Z-bearings 21, 22, 23, 31, 32, 33 andX-bearings 24, 25, 34, 35 may be, for example, gas bearings. The balancemasses 20, 30 are thus free to move in all three translational degreesof freedom and so provide balancing to the mask table in thosedirections. Rotational balancing in Rx and Ry is provided because theZ-bearings 21, 22, 23, 31, 32, 33 can be moved independently and arespaced apart. Balancing for Ry movements is provided by differentiallydriving the two balance masses 20, 30, as is discussed below.

[0067] If the ranges of movement of the mask table in the degrees offreedom other than Y translation are small, as is the case in thepresent embodiment, the necessary freedom of movement of the balancemasses can also be accommodated by leaf spring arrangements, compliantbearings or other stiff bearings in combination with a gravitycompensator. It is also possible to arrange that reaction forces in someor all of the other degrees of freedom are only transmitted to one ofthe balance masses so that only that balance mass needs to be supportedwith controlled compliance in the relevant degrees of freedom.

[0068] The stiffness of the bearings or supports in the other degrees offreedom and the mass of the balance mass(es) form a mass-spring systemthat acts as a low-pass filter, i.e. only low frequency forces aretransmitted to the machine frame. Significant attenuation of thereaction forces can be obtained if the natural frequency of thismassspring system is substantially, for example 5 to 50 times, lowerthan the fundamental frequency of the actuation forces.

[0069] As will be described below, the mask table MT is driven byactuators acting against the balance masses 20, 30 so that theyaccelerate in the opposite direction to the mask table MT. Themagnitudes of the accelerations of the balance masses and the mask tableMT will be proportional to their masses and so the ranges of movement ofthe balance masses and the mask table in the various directions must bein the ratio of their masses. To reduce the ranges of movement that mustbe provided for the balance masses 20, 30 to accommodate the desiredranges of movement of the mask table MT, the balance masses 20, 30 aremade relatively massive, e.g. each 2 to 10 times the mass of the masktable MT. The centers of mass of the balancing masses 20, 30 and masktable MT are preferably as close as possible in the Z-direction, e.g.substantially less than 100 mm, in order to minimise pitching or rollingmoments.

[0070] In the present embodiment, the mask table MT is driven in theY-direction by Y1-drive 18 acting between it and balance mass 20 andY2-drive 17 acting between it and balance mass 30. Y1- and Y2-drives 17,18 may, for example, comprise linear motors with an armature mounted tothe mask table MT and an elongate stator mounted to the respectivebalance mass. Yi-drive exerts, in operation, a force F_(yi) on the masktable MT and an equal and opposite reaction force R_(yi) on therespective balance mass.

[0071] Positioning in the X-direction is effected by a single X-actuator19 acting against balance mass 30. X-actuator 19 may also be a linearmotor with armature mounted to the mask table and stator mounted to thebalance mass or may be an elongated voice-coil motor free to displace inthe Y-direction, or a cylindrical voice-coil motor coupled to anaerostatic bearing that bears against a surface parallel to the YZplane. To enable the mask table to be driven in the X-direction whateverthe relative Y position of the mask table MT and balance mass 30, ifX-actuator is a linear motor, the stator must extend the whole of thecombined range of movement of the balance mass and mask table in the Ydirection. The line of action of the X-actuator 19 is preferablyarranged to pass through at least the Y-position of the center ofgravity CG_(MT) of the mask table MT so as to minimise the generation ofRz moments.

[0072] It follows from Newton's laws that if there is no rotationalmovement of the mask table, the displacements Δy_(b1), ΔY_(b2) andΔY_(MT) of the balance masses 20, 30 and mask table MT satisfy thefollowing conditions: $\begin{matrix}{{\frac{\Delta \quad y_{MT}}{\Delta \quad y_{b\quad 1}} = {{- \frac{m_{b\quad 1}}{m_{MT}}} \cdot \frac{l_{1} + l_{2}}{l_{2}}}};{\frac{\Delta \quad y_{MT}}{\Delta \quad y_{b\quad 2}} = {{- \frac{m_{b\quad 2}}{m_{MT}}} \cdot \frac{l_{1} + l_{2}}{l_{2}}}}} & \lbrack 1\rbrack\end{matrix}$

[0073] where:

[0074] and l₁ and l₂ are respectively the distances in the X-directionbetween the centers of gravity CG_(B1), CG_(B2) of the balance masses20, 30 and the center of gravity CG_(MT) of the mask table MT; and

[0075] m_(b1), m_(b2) and m_(MT) are the masses of the balance masses20, 30 and mask table MT.

[0076] If m_(b1)=m_(b2)=m_(b) and l₁=l₂, then equation 1 can be reducedto: $\begin{matrix}{{\Delta \quad y_{b\quad 1}} = {{\Delta \quad y_{b\quad 2}} = {{- \Delta}\quad {y_{M\quad T} \cdot \frac{m_{M\quad T}}{2\quad m_{b}}}}}} & \lbrack 2\rbrack\end{matrix}$

[0077] To effect a yawing (Rz) movement of the mask stage whilst stillcontaining the reaction forces within the balance mass system, theforces applied by Y1- and Y2-drives 17, 18 are controlled to takeadvantage of D'Alambert forces by moving the balance masses in oppositedirections. Note that if the yawing motion is effected at the same timeas a movement in Y, the balance masses may move in the same directionbut by differing amounts, thus the movement in opposite directions isrelative rather than absolute. For a counter-clockwise movement of themask stage by an angle θ_(MT) the necessary relative movements of thebalance masses are given by: $\begin{matrix}{{{\Delta \quad y_{b\quad 1}} = {- \frac{J_{MT} \cdot \theta_{M\quad T}}{( {l_{1} + l_{2}} ) \cdot m_{1}}}};{{\Delta \quad y_{b\quad 2}} = {- \frac{J_{MT} \cdot \theta_{MT}}{( {l_{1} + l_{2}} ) \cdot m_{2}}}}} & \lbrack 3\rbrack\end{matrix}$

[0078] where J_(MT) is the moment of inertia of the mask table MT.

[0079] It should be noted that the present invention does not requirethe masses of the first and second balance masses to be equal nor thatthey be disposed equidistantly about the centre of gravity of the masktable.

[0080] In a perfect, closed system, the combined center of mass of themask table MT and balance masses 20, 30 will be stationary, however itis preferable to provide a negative feedback servo system to correctlong-term cumulative translations (drift) of the balance masses thatmight arise from such factors as: cabling to the mask table and drives,misalignment of the drives, minute friction in the bearings, theapparatus not being perfectly horizontal, etc. As an alternative to theactive drift control system described below, a passive system, e.g.based on low-stiffness springs, may be used.

[0081]FIG. 4 shows the control loop of the servo system 130. The Y andRz setpoints of the balance masses with respect to the machine frame aresupplied to the positive input of subtractor 131, whose output is passedto the servo controller 132. Feedback to the negative input ofsubtractor 131 is provided by one or more multiple-degree-of-freedommeasurement systems 134 which measure the positions of the balancemasses and driven mass (mask table). The servo controller controls atwo-degree-of-freedom actuator system 133 which applies the necessarycorrections to the balance masses 20, 30. The positions of both balancemasses and driven mass may be measured relative to a fixed frame ofreference. Alternatively, the position of one, e.g the balance masses,may be measured relative to the reference frame and the position of thedriven mass measured relative to the balance masses. In the latter casethe relative position data can be transformed to absolute position dataeither in software or by hardware. Particularly in the Y-direction, theposition measurement may be performed by a linear encoder with a hightolerance to residual relative movements in the other degrees offreedom, such as those described in U.S. Pat. No. 5,646,730, forexample.

[0082] The set points of the servo system 130 are determined so as toensure that the combined center of mass of the mask table MT and balancemasses 20, 30 remains unchanged in the X, Y, Rz plane. This defines thecondition:

m _(MT) ·{right arrow over (u)} _(MT)(t)+m _(b1) ·{right arrow over (u)}_(b1)(t)+m _(b2) ·{right arrow over (u)} _(b2)(t)=m _(MT) ·{right arrowover (u)} _(MT)(O)+m _(b1) ·{right arrow over (u)} _(b1)(O)+m _(b2)·{right arrow over (u)} _(b2)(O)  [4]

[0083] where {right arrow over (u)}_(i)(t) is the vector position ofmass i in the X-Y plane at time t relative to a fixed reference point.The error signal between the calculated (using equation [4]) andmeasured positions is provided to the actuation system 133 which appliesappropriate correction forces to the balance masses 20, 30. The lowestresonance mode of the balancing frame and/or machine base is preferablyat least a factor of five higher than the servo bandwidth of the driftcontrol system.

[0084] The above described servo system can be used in the Y-directiononly with drift control in the other degrees of freedom being performedby the low stiffness of the supports for the balance masses in thosedegrees of freedom.

[0085] Embodiment 2

[0086] A second embodiment of the invention is shown in FIGS. 5 and 6and is essentially the same as the first embodiment except as notedbelow.

[0087] The second embodiment is particularly applicable to lithographicapparatus employing reflective masks so that the space underneath themask table MT does not need to be kept clear. Advantage is taken of thisfact to support the mask table MT over a third balance mass 60. Thirdbalance mass 60 has a planar, horizontal upper surface over which isguided the mask table MT supported by bearings 71, 72, 73. Thesebearings may be, for example, gas bearings. Third balance mass 60 is inturn supported over the machine base frame by compliant bearings 61, 62,63, which may comprise low stiffniess springs. The third balance massdoes not move in the XY plane so can alternatively be supported by leafsprings or gas cylinders without actual bearings. As illustrated, thesecond embodiment uses cylindrical voice coils 74, 75 in combinationwith X-bearings 76, 77 acting against the side of the second balancemass 30 for X-direction actuation. The X-bearings 76, 77 may be opposedpad bearings or preloaded so that forces in both directions can beexerted.

[0088] Embodiment 3

[0089] In a third embodiment, shown in FIGS. 7 and 8 and which is thesame as the first embodiment save as described below, the longstrokemodule positions a short, stroke frame 80 in Y and Rz only. Mask tableMT is driven relative to the short stroke frame 80 to position the maskin six degrees of freedom to a high precision. Such positioning iseffected by short stroke Z-actuators 81, 82, 83, X-actuator 84 andY-actuators 85, 86. The short stroke frame 80 is supported over firstand second balance masses 20, 30 by stiff Z-bearings 14′, 15′ 16′, whichmay be gas bearings acting on the planar upper surface of the balancemasses. Short stroke frame 80 is also constrained in X by bearing 78relative to only one of the balance masses, in this case the secondbalance mass 30.

[0090] In the Y and Rz directions, the mask table MT moves with theshort, stroke frame 80 so that in equations 2 and 3 the mass and momentof inertia, m_(MT) and J_(MT), should be replaced by the combined massand moment of inertia of the mask table MT and short stroke frame 80.However, in the other degrees of freedom the short stroke frame 80 isconstrained to move with the balance mass and so increases the effectivebalancing mass, reducing its stroke. The center of gravity of the masktable MT is preferably coplanar, or close to coplanar, with that of theshort stroke frame 80 and balance masses 20, 30.

[0091] Embodiment 4

[0092] A cable ducting device according to a fourth embodiment of theinvention is shown in FIGS. 9 and 9A. Two cable ducts 151 a, 51 b areused to carry cables and other conduits for utilities, such as controlsignals and power, required by the mask table. The two cable ducts 151a, 151 b are laid out in opposite directions between a terminal 152mounted on the mask table and a terminal 153 mounted on the machineframe so that as the mask table moves in the Y direction, one cable ductis rolling up and the other is unrolling. The total length of cable ductmoving with the mask table therefore remains constant, whatever the Yposition of the mask table. The moving mass therefore remains constant.Also, any residual tendencies of the cable ducts to roll up or unrollwill counteract each other. The cable ducts 151 a, 151 b have a slightlycurved cross-section, shown in FIG. 9A which is a cross-sectional viewalong the line A—A, in the same manner as a measuring tape. Thisprevents sagging and helps maintain a neat “U-shape” as the mask tablemoves.

[0093] Whilst we have described above specific embodiments of theinvention it will be appreciated that the invention may be practicedotherwise than described. The description is not intended to limit theinvention. In particular it will be appreciated that the invention maybe used in the reticle or mask table of a lithographic apparatus and inany other type of apparatus where fast and accurate positioning of anobject in a plane is desirable.

[0094] Although this text has concentrated on lithographic apparatus andmethods whereby a mask is used to pattern the radiation beam enteringthe projection system, it should be noted that the invention presentedhere should be seen in the broader context of lithographic apparatus andmethods employing generic “patterning means” to pattern the saidradiation beam. The term “patterning means” as here employed refersbroadly to means that can be used to endow an incoming radiation beamwith a patterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate; the term “light valve” hasalso been used in this context. Generally, the said pattern willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit or other device.Besides a mask on a mask table, such patterning means include thefollowing exemplary embodiments:

[0095] A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which areincorporated herein by reference.

[0096] A programmable LCD array. An example of such a construction isgiven in U.S. Pat. No. 5,229,872, which is incorporated herein byreference.

1. A lithographic projection apparatus comprising: a radiation systemwhich supplies a projection beam of radiation; a first object tableconstructed and arranged to hold a mask; a second object tableconstructed and arranged to hold a substrate; an imaging projectionsystem configured to image irradiated portions of the mask onto targetportions of the substrate; first and second balance masses disposedalong opposite sides of the first object table; and first and secondmotors configured to move the first object table, each motor having twocooperating electromagnetic members, a first of the members beingmounted to the first object table and a second of the members beingmounted to at least one of the first and second balance masses, whereinsaid first and second balance masses are substantially free to move inat least a first direction to provide balancing to the first objecttable.
 2. The apparatus of claim 1, wherein said first direction issubstantially parallel to a direction of motion of the first objecttable.
 3. The apparatus of claim 1, wherein said first and secondbalance masses have parallel planar upper surfaces.
 4. The apparatus ofclaim 3, wherein the first object table is provided with bearings, saidbearings being configured to act on said planar surfaces to allow saidfirst object table to move substantially without friction relative tothe first and second balance masses.
 5. The apparatus of claim 4,wherein said bearings include actuators configured to move said firstobject table in a direction substantially perpendicular to said firstdirection and to rotate said first object table relative to at leastsaid first direction.
 6. The apparatus of claim 1, further comprises: abase structure; and parallel rails coupled to said base structure, saidparallel rails having substantially flat horizontal upper surfaces,wherein said first and second balance masses are supported in a seconddirection substantially perpendicular to said first direction by aplurality of bearings acting upon said substantially flat horizontalupper surfaces.
 7. The apparatus of claim 6, wherein said first and saidsecond balance masses are free to move in said first direction over arelatively wide range of motion.
 8. The apparatus of claim 6, whereinsaid first and second balance masses are free to move in a thirddirection substantially perpendicular to said first and said seconddirections, and the movement of said first and second balance masses inthe third direction is provided by compliant bearings acting againstsubstantially vertical walls of said parallel rails.
 9. The apparatus ofclaim 6, wherein independent movement of the bearings acting upon thehorizontal upper surfaces of the parallel rails provides rotationalmovement of said first and second balance masses around said firstdirection and said third direction.
 10. The apparatus of claim 6,wherein said bearings have a low stiffness in said second direction suchthat at least one of said first and said second balance masses issubstantially free to move in said second direction.
 11. The apparatusof claim 1, wherein a mass of each of said first and second balancemasses is 2 to 10 times larger than a mass of said first object table.12. The apparatus of claim 1, wherein a center of gravity of said firstand second balancing masses and a center of gravity of said first objecttable are located at less than 100 mm apart from each other in adirection perpendicular to said first direction.
 13. The apparatus ofclaim 1, wherein said first object table is driven in said firstdirection by the first motor acting between the first object table andthe first balance mass and by the second motor acting between the firstobject table and the second balance mass.
 14. The apparatus of claim 1,wherein the first and second motors comprise linear motors, armaturesmounted to the first object table, and an elongate stator mounted toeach of the first and second balance masses.
 15. The apparatus of claim13, wherein said first object table is movable in a directionsubstantially perpendicular to said first direction by a third motor,and a line of action of said third motor passes through at least aposition of a center of gravity of said first object table in the firstdirection.
 16. The apparatus of claim 1, further comprising a driftcontrol which limits drift of the first and second balance masses. 17.The apparatus of claim 16, wherein the drift control comprises a servocontrol system and an actuator which applies forces to the first andsecond balance masses biasing a combined center of gravity of the firstand second balance masses and the first object table to a desiredposition.
 18. The apparatus of claim 17, wherein the drift control has aservo bandwidth at least a factor of five lower than a lowest resonancefrequency of the first and second balance masses and a base of theapparatus.
 19. The apparatus of claim 16, wherein the drift controlcomprises an active system.
 20. The apparatus of claim 16, wherein thedrift control comprises a negative-feedback servo system.
 21. Theapparatus of claim 16, wherein the drift control comprises a passivesystem.
 22. The apparatus of claim 16, wherein the drift controlcomprises at least one spring.
 23. The apparatus of claim 1, furthercomprising a third balance mass having a substantially planar uppersurface, wherein said first object table is positioned over saidsubstantially planar surface of said third balance mass.
 24. Theapparatus of claim 1, further comprising a short stroke frame positionedover said first and said second balance masses and supported by aplurality of bearings.
 25. The apparatus of claim 24, wherein said shortstroke frame is movable in the first direction and second directionperpendicular to said first direction and rotatable around a thirddirection perpendicular to said first and said second directions. 26.The apparatus of claim 24, wherein the first object table is drivenrelative to said short stroke frame to position said mask in six degreesof freedom.